Glasses, thermal conductivity values

Polyurethane. Polyurethanes (pu) are predominantly thermosets. The preparation processes for polyurethane foams have several steps (see Urethane polymers) and many variations that lead to products of widely differing properties. Polyurethane foams can have quite low thermal conductivity values, among the lowest of all types of thermal insulation, and have replaced polystyrene and glass fiber as insulation in refrigeration. The sprayed-on foam can be appHed to walls, roofs, tanks, and pipes, and between walls or surfacing materials directly. The slabs can be used as insulation in the usual ways. 

Thermal conductivity of the obtained aerogel glass materials was emalyzed by using a Hot Disk Thermal Constants Analyzer (Model TPS 2500S). A disk-type Kapton Sensor 5465 with radius 3.189 mm was used. The temperatrrre increase of the samples as a function of time was recorded to compute the thermal conductivity. The fined thermal conductivity value reported here was the arithmetic mean of three individual measurements trader different measurement conditions (heating power 5-20 mW, measurement time 5 0 s). 

Board material The FR-4 versus polyimide board material properties did not have a significant impact on trace temperature, which is determined primarily by the thermal conductivity of the dielectric laminate material construction. Table 16.2 lists measured thermal conductivity values for each of the test boards.The column labeled kz presents values through the thickness of the board and represents the resin thermal conductivity.The values in columns kx and ky are in-plane and the difference is attributed to the influence of the glass fiber. 

Alumina porcelains contain corundum and glass phases. Both phases are continuous at a composition corresponding to about 9 vol% glass. In such a case, the thermal conductivity value will be in between that of the two phases. Generally, the glass is continuous in vitreous ceramics. Therefore, the conductivity of these materials is closer to that of the glass phase. 

Thermal conductivity values for a number of ceramic materials are given in Table 19.1 room-temperature thermal conductivities range between approximately 2 and 50 W/m K. Glass and other amorphous ceramics have lower conductivities than crystalline ceramics because the phonon scattering is much more effective when the atomie structure is highly disordered and irregular. 

Glass 0.04 Other Non-Fibrous Materials Aluminum 205 Steel 16-60 Water 0.58 Ice 2.22 Snow 0.05-0.25 Air 0.024 Most non-fibrous materials are isotropic and only have one thermal conductivity value. ... 

Searching product specifications on the internet (from glass manufacturers and suppliers websites), a number of values for thermal conductivity for alkali borosilicate glasses have been quoted. It seems that below the glass transition temperature, values for thermal conductivity are quoted around 1.0 0.25 W m K at around room temperature, increasing to a value of about 2.5 0.25 W m K at about 1400 K. These values seem reasonably insensitive to composition and so may be used as markers for our glass thermal conductivity. 

The simulated value for thermal diffusivity for this glass composition at 1050°C was found to be 0.615 m s. By substituting this in Equation 21.2 and using the literature values for density and heat capacity, the derived thermal conductivity value is 2.72 W m- K-. ... 

The pure resins are limited to light loads and low draft velocities because of cold-flow problems and low thermal conductivity values (k). Therefore, various fillers are added such as glass, graphite, bronze, and These resins and the filled compounds have a pressure-velocity limit of 8000-10,000 psi fpm. Unfilled PTFE has a static p = 0.05-0.08, and a dynamic p = 0.10-0.13. The p values are larger with fillers ranging from 0.08 up to 0.50. The coefficient of friction, p decreased with pressure under dynamic conditions. 

Siace the pores ia an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is iaefficient. Coupled with the fact that sohd conduction is suppressed due to the low density, a siUca aerogel has a typical thermal conductivity of 0.015 W/(m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chloro uorocarbon)-blown polyurethane foams (54). 

Phonon transport is the main conduction mechanism below 300°C. Compositional effects are significant because the mean free phonon path is limited by the random glass stmcture. Estimates of the mean free phonon path in vitreous siUca, made using elastic wave velocity, heat capacity, and thermal conductivity data, generate a value of 520 pm, which is on the order of the dimensions of the SiO tetrahedron (151). Radiative conduction mechanisms can be significant at higher temperatures. 

The thermal conductivity of solids has a wide range of numerical values, depending upon whether the solid is a relatively good conductor of heat, such as metal, or a poor conductor, such as glass-fiber or calcium silicalc. The laUer serves as insulation. 

For certain products, skill is required to estimate a product s performance under steady-state heat-flow conditions, especially those made of RPs (Fig. 7-19). The method and repeatability of the processing technique can have a significant effect. In general, thermal conductivity is low for plastics and the plastic s structure does not alter its value significantly. To increase it the usual approach is to add metallic fillers, glass fibers, or electrically insulating fillers such as alumina. Foaming can be used to decrease thermal conductivity. 

The value of the coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and the cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths. In solids, heat is normally transferred by conduction some materials such as metals have a high thermal conductivity, whilst others such as ceramics have a low conductivity. Transparent solids like glass also transmit radiant energy particularly in the visible part of the spectrum. 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

The transmission of heat is favored by the presence of ordered crystalline lattices and covalently bonded atoms. Thus graphite, quartz, and diamond are good thermal conductors, while less-ordered forms of quartz such as glass have lower thermal conductivities. Table 7.3 contains a brief listing of thermal conductivities for a number of materials. Most polymeric materials have X values between 10 and 10° W m- K"1. 

The volume coefficient of expansion of Teflon AF is linear with temperature and quite low. The coefficients are 280 ppm/°C and 300 ppm/°C for AF-1600 and AF-2400, respectively. Above the glass transition temperature these values increase sharply. Thermal conductivity is quite low, increasing from only 0.05W/mK at 40°C to 0.2 W/mK at 260°C. Many of these properties are believed to be related to the very low (1.7-1.8 g/ml) densities of these dioxole... 

The filled skutterudite antimonides appear to represent excellent examples of electron-crystal, phonon-glass materials. The incoherent rattling of the loosely bound lanthanide atoms in these materials is inferred from the large values of the ADP parameters obtained in single-crystal structure refinements. This rattling lowers the thermal conductivity at room temperature to values within two to three times Km... 

The thermophysical properties, such as glass transition, specific heat, melting point, and the crystallization temperature of virgin polymers are by-and-large available in the literature. However, the thermal conductivity or diffusivity, especially in the molten state, is not readily available, and values reported may differ due to experimental difficulties. The density of the polymer, or more generally, the pressure-volume-temperature (PVT) diagram, is also not readily available and the data are not easily convertible to simple analytical form. Thus, simplification or approximations have to be made to obtain a solution to the problem at hand. 

The thermal conductivities of various insulating materials are also given in Appendix A. Some typical values are 0.038 W/m °C for glass wool and 0.78 W/m °C for window glass. At high temperatures, the energy transfer through... 

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